1. Field of the Invention
The present invention relates to a laser light irradiation apparatus and a laser light irradiation method, and particularly relates to a laser light irradiation apparatus and a laser light irradiation method using a beam expander optical system.
2. Description of the Related Art
Recently, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has been drastically advanced and developed for being applied to an active matrix display device. In particular, a TFT using a polycrystalline semiconductor film has a higher electron field-effect mobility (also referred to as a mobility) than a conventional TFT using an amorphous semiconductor film; accordingly, high-speed operation is possible. Therefore, although the control of a pixel has been conventionally carried out by a driver circuit which is provided outside a substrate, the control can be carried out by a driver circuit which is formed over the same substrate as the one over which a pixel is formed.
As a substrate used for a semiconductor device, a glass substrate is expected more than a quartz substrate and a single-crystalline semiconductor substrate in terms of cost. The glass substrate has poor heat resistance and is easily deformed by heat. Therefore, when a semiconductor film is crystallized in order to form a TFT using a polycrystalline semiconductor film over a glass substrate, a method for crystallizing a semiconductor film by being irradiated with a laser beam is often used to avoid thermal deformation of the glass substrate.
Features of the crystallization of a semiconductor film by laser light are that, compared with an annealing method utilizing radiation heating or conductive heating, processing time can be drastically reduced, a semiconductor substrate or a semiconductor film over a substrate is selectively or locally heated so that the substrate is hardly thermal-damaged, and the like.
Generally, laser light oscillated from a laser oscillator has a Gaussian spatial intensity distribution. Therefore, in the case where an irradiation object is directly irradiated with laser light oscillated from a laser oscillator, the energy distribution varies in an irradiated region. For example, when crystallization or improvement of film quality is conducted by irradiating a semiconductor film of silicon or the like with laser light, if the semiconductor film is directly irradiated with laser light having a Gaussian spatial intensity distribution, in a central portion and an edge portion of an irradiated region, the energy distribution is different so that melt time of the semiconductor film varies. Consequently, crystallinity of the semiconductor film becomes nonuniform, and a semiconductor film having a desired characteristic cannot be obtained.
Accordingly, in general, after the spatial intensity distribution of laser light oscillated from a laser oscillator is uniformed by using some kind of laser light shaping means, an irradiation object is irradiated with the laser light. For example, as the laser light shaping means, a beam expander optical system is widely used (for example. Reference 1: Japanese Published Patent Application No. H7-41845).